San Francisco 49ers cornerback Nnamdi Asomugha (24) walks on the sideline during the fourth quarter of an NFL preseason football game against the Minnesota Vikings in San Francisco, Sunday, Aug. 25, 2013. (AP Photo/Marcio Jose Sanchez)

Those early fears have been allayed: Nnamdi Asomugha won't require a walker or a hearing aid this season.

The 49ers' cornerback played like a past-his-prime player in Philadelphia last season, and defensive coordinator Vic Fangio questioned whether Asomugha had "gas left in his tank" early in training camp.

Asomugha, 32, has since done more than just putter along in the preseason.

In three exhibition games, Asomugha has allowed one catch for 5 yards in 39 snaps in pass coverage, according to Pro Football Focus. Quarterbacks have tossed four passes in his direction and have a 39.6 passer rating when targeting him.

As a result, Old Man Asomugha will have a new title when the 49ers open the regular season against Green Bay on Sept. 8: No. 3 cornerback.

Fangio said Asomugha has overtaken Tramaine Brock for that role, which had been filled by Chris Culliverbefore he sustained a season-ending ACL tear in training camp. Asomugha jumped ahead while Brock was dealing with an injury the past few weeks. Asomugha, who wore a noncontact jersey in practice Tuesday, is himself at less than full strength.

Still, Fangio said he has won the No. 3 spot.

"He's done well in his reps that he's had in the game and in practice," Fangio said. "Particularly the last two or three weeks, he's played much better."

Asomugha isn't a starter, but he'll be a major contributor. Last season, Culliver played 64 percent of the defensive snaps. This season, the No. 3 cornerback probably will log similar time as the 49ers face a schedule that includes nine games against quarterbacks who finished among the top 10 in passer rating last season.

Fangio said he doesn't plan to monitor Asomugha's snaps because of his age. The 10-year veteran clearly has answered some of the questions Fangio had during training camp.

"He's still playing at a good enough level to be a major part of our defense," he said.

If Asomugha falters, the 49ers could turn to Brock or, later in the season, Eric Wright, who reported to the team this week and will spend the first six weeks of the season on the non-football injury list. Wright, 28, is a six-year veteran who has made 81 starts since he was a second-round pick in 2007.

He recently was viewed as a top-end cornerback - he signed a five-year, $37.5 million deal with Tampa Bay last year - but has since struggled on and off the field. He was signed Aug. 8 but was away from the team while addressing what general manager Trent Baalketermed a "personal matter."

The 49ers, of course, hope Asomugha recaptures his Pro Bowl form so they won't need to consider Plan B.

Asomugha, who began the summer competing for a roster spot, said he's not surprised by his ascent. He knew he wasn't ready for the early-bird special just yet.

"I've just been going out and just working each day," he said. "I know my abilities, I know I can help the team. ... It's just letting everything unfold, playing as best as I could."

Backup bonanza: Fangio said the 49ers would rest a "good bit" of their defensive starters in Thursday's preseason finale at San Diego. He indicated rookie safety Eric Reid would be among those who wouldn't play, noting the first-round pick has played 116 snaps in the first three preseason games.

On Sunday, Jim Harbaugh said quarterback Colin Kaepernickprobably will play. In last year's preseason finale against the Chargers, the offensive linemen were the only members of the first-string offense to play one full drive.